This invention relates to a servo mechanism which may be used in an automatic speed control system for a vehicle, such as a heavy diesel truck, having a source of air pressure aboard.
Electrical speed sensing devices permit accurate speed measurements and are capable of providing signals which call upon the actuators to move in one direction or another depending upon the measured speed. When such electrical speed sensing devices are coupled to a proper actuator a speed control for a vehicle can result. The reliability, response and dependability of the speed control system depends greatly upon the actuator.
One problem in providing a speed control system on a diesel truck as compared with a common passenger car is that trucks are relatively underpowered and do not have the capability to accelerate rapidly in order to maintain a constant rate of speed under varying load and road conditions.
Common passenger car speed controls utilize the vacuum created in the intake manifold of an engine to provide power for operating a throttle mechanism. The vacuum thus produced varies with engine load and atmospheric pressure so is not a reliable power source for use in more sophisticated apparatus.
Attempts have been made to solve the problem of vacuum powered actuators for use on vehicle speed controls by applying vacuum from the intake manifold to one side of the servo diaphram and applying pressure from the exhause manifold to the other side of the diaphram, but due to the construction of a diesel engine and the inherent complications of installation there has been little satisfaction derived from this attempted solution to the vacuum powered servo problem.
One system has been devised whereby the compressed air available on most heavy trucks is used to operate a hydraulic cylinder which, in turn, operates an engine throttle. This system is subject to response variation due to temperature and air/oil polution. It is also complicated when compared to the vacuum-servo actuated systems.
Speed control of engines by means of a mechanical governor has entailed complicated apparatus and have exhibited limited sensitivity.